PROCEEDINGS 



OF THE 



Cambritrgc ^Ijibsnpljkal j&rrictjj. 



January 29, 1883. 



Mr Glaisher, President, in the chair. 



Messrs C. V. Coates, M. A., Trinity College, and D. W. Sam ways, 

 B.A., St John's College, were balloted for and duly elected Fellows 

 of the Society. 



The following communications were made to the Society: — 



(1) On the isochromatic curves of polarized light seen in a 

 uniaxal crystal cut at right angles to the optic axis. By R. T. 

 Glazebrook, M.A., F.R.S. 



When a thin plate of Iceland spar is viewed between two 

 Nicols in a convergent beam of homogeneous light it is usually 

 stated that the field is crossed by two rectangular brushes of 

 uniform intensity p 2 cos 2 7, p 2 being the intensity of the incident 

 beam and 7 the angle between the principal planes of the two 

 Nicols, while between these brushes are seen a series of circles of 

 uniform intensity — the isochromatic curves are said to be circles. 



It has frequently seemed to me that the coloured figures gene- 

 rally given in the text-books failed to represent the appearances 

 actually observed, and recently while writing an account of the 

 phenomena for my book on Physical Optics the reason of this 

 failure occurred to me. The curves of equal intensity are not 

 circles at all. Let us take as origin the point in which the axis 

 of the pencil cuts the plate. Let be the azimuth of any point P 

 (Fig. 1), reckoned from OY, the direction of vibration of the 



VOL. IV. TT. vi. 22 



